Random access transmissions can reduce signaling overhead and the transmission delay for up-link (UL) access. With random access transmissions, UL communication is unscheduled, making it more efficient due to elimination of scheduling requests sent to a base station for UL communication.
Sparse code multiple access (SCMA) and low-density signature (LDS) are two multiple-access techniques that may be used in random access transmissions. LDS is a version of code division multiple access (CDMA), which is a channel access method that allows several transmitters, e.g., users, to share a radio channel by applying a spreading code, or spreading signature, to modulate each transmitter's signal. LDS uses a low-density spread code to spread quadrature amplitude modulated (QAM) symbols in the time and frequency domains. SCMA enhances LDS and CDMA by introducing a codebook-based multi-dimensional spreading encoder rather than using a spreading signature.